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LifeNet Health Spinal Implant Delivery
Device
Nan Cheng1
Ryan Foley1
Alex Long1
John Majewski1
Michael Francis PhD2
1) University of Virginia – All of equal contribution
2) LifeNet Health
2. 2
Abstract
This paper describes the development process of a lightweight, compact, and
multifunctional delivery device for a spinal implant during a TLIF spinal fusion surgery.
Previous technologies are vast and diversified in terms of delivery functions, yet this device is
the first to incorporate putty delivery in the same device. A list of initial ideas was evaluated
based on the needs and specifications we generated. A final concept, the Tri-Grip, featured three
gripping metal blades, one on the top and two on the bottom. We selected the design for its
stability in holding the implant and its ability to distract the disc space. Following the formation
of this concept, we developed a controlling mechanism, a handle, and a putty delivery system.
The implant is gripped by a plunger that is controlled by two buttons on the handle for back and
forth movements. The putty is packed into a cartridge that is connected to the hollow inside of
the plunger and is pushed out by a ramrod. This mechanism significantly simplifies the previous
separate steps of implant and putty delivery and reduces operation time. Based on spinal surgeon
feedbacks, we made iterative improvements to the design. The end result is a partially functional
second generation prototype. For future work, potential improvements include modifying handle
ergonomics and tip orientation and researching new areas of application for this device.
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Figure 1 - Lumbar Spinal Fusion Variations. Above are the four most common variations of lumbar
spinal fusion surgery. The direction of the incision is indicated by the arrow and any native tissue
blocking access to the disc is not shown. Though all arrows are pointing downward to some degree,
the arrows are entering posteriorly in (a) and (b) and anteriorly in (c) and (d). Image was adopted
from Davis et al., Modern Spinal Instrumentation.
Introduction
Lumbar spinal surgery is one of the most common back related surgeries with 122,316
lumbar spinal fusions for degenerative conditions performed in 2001 (Deyo, 2005). From 1998
to 2008, the number of lumbar spinal fusions increased by 137 percent; associated expenditures
experienced a 7.9 fold increase from $4.3 billion to $33.9 billion (Miller, L., 2012). The two
major related diseases are herniated disc and degenerative disc disease (DDD). A herniated disc
occurs when the spinal disc becomes damaged or slips out of place due to trauma; the slipped
disc can press against a nerve, often resulting in pain. Lumbar DDD occurs as a normal part of
the aging process and results when the spinal disc loses its shock absorbing abilities due to loss
of fluid. The surgical treatment of these two problems includes discectomy, or removal of the
original disc, and fusion of the two adjacent vertebrae.
There are multiple minimally invasive procedures for lumbar fusion surgery. The main
approaches are anterior lumbar interbody fusion (ALIF), extreme lateral interbody fusion
(XLIF), transforaminal lumbar interbody fusion (TLIF), and posterior lumbar interbody fusion
(PLIF). Each result in the same outcome, namely a spinal implant intended to fuse the vertebrae.
The most notable difference between the different approaches is the direction from which the
surgeon approaches the spine, as illustrated in Figure 1. All offer advantages with regards to the
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anatomy, but a TLIF creates a balance of risk for the barriers to the disc, such as the aorta
anteriorly, the muscles laterally, and the nerves posteriorly. With a TLIF, the surgeon makes an
incision on the patient’s lower back slightly to the left or right of center. The surgeon can then
approach the damaged disc by shaving off small bits of bone around the foramen, or where the
peripheral nerve exits the spinal column.
The aim of this project was to design a delivery device for spinal implants developed at
LifeNet Health. In addition to inserting a solid implant, we strived to deliver a putty-like
substance to the disc space that contains growth factors to help facilitate fusion between adjacent
discs. Many physicians already incorporate putty into the surgery, though they do so with
separate devices. We wished to create an all-inclusive device that can accommodate implants of
variable sizes and putty delivery without removing the device from the operating field. In
addition, we planned to design a minimally invasive device in order to reduce the recovery time
for patients and minimize the size of scars following the surgery.
Prior Art
The prior art for lumbar fusion implant inserters is vast. While the surgery dates back to
the 1920s, dedicated surgical instruments were not invented until much later. In 1969, the first
orthopedic surgical instrument designed to retract and insert an implant was described (Morrison,
1969). The patent is vague and does not describe specific mechanisms for delivering an implant
into the spine. As the surgery gained prevalence, surgeons recognized the need to accommodate
a range of intervertebral space sizes (Ripple, 1986). Subsequent improvements have been aimed
at describing novel ways of delivering the implant. One area of innovation is the proximal end of
the device where the surgeon maneuvers the instrument; in some embodiments, this handle
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doubles as an active inserter for the implant. The prior art ranges from devices that include a
squeezable actuator (Reindel, 2009; Zalenski, 2012) to screw and gear mechanisms that ratchet
the implant into the intervertebral space (Castro, 2002; Miller, P., 2012). At the distal end,
various mechanisms have been developed for the intervertebral space. Many incorporate
distractors to hold the upper and lower vertebrae open in order to allow easy insertion of the
implant into the disc space (Castro, 2002; Fraser, 2012). Several inventors crafted designs of
complimentary inserters and implants to support a stronger intellectual property claim and enable
for an attractive product package offering later (Curran, 2012; Spann, 2012). Other inventions
were designed to account for specific anatomical landmarks. For example, a device with an
articulating joint between the proximal and distal ends allows for the implant to be negotiated
around risky areas of the spine (Koulisis, 2008). Another device was designed for lateral
insertion and allows the surgeon to rotate the implant in place (Spann, 2012). Specifically in the
past ten years, devices have skewed towards a minimally invasive approach integrated into the
design. Several patents claim a telescoping shaft to guide the implant to the disc space while
avoiding the surrounding muscle and nerves (Castro, 2002; Spann, 2012). This shaft allows the
surgeon to work from a distance and create an incision just large enough for the port.
Contribution
At this point, no device designed for a TLIF is able to deliver both an implant and putty.
Putty delivery is done very archaically, by simply packing the putty into the disc space through a
tube. In this paper, we discuss our development of a single device that can be used to deliver
both the implant and putty, thus minimizing the number of tools needed for the surgery.
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Defining Device Specifications
The initial objective of the project was to design a delivery device to insert the implants
and putty developed by LifeNet Health. We decided to strive for a minimally invasive tool in
order to follow the trend of surgical procedures straying away from large incisions, although our
device would still be able to accommodate a physician that preferred a more open approach. As
mentioned previously, we focused on the TLIF, however we did not rule out the possibility of a
universal device for all of the procedures in future iterations. The TLIF approach was the best
starting point because of its risk balance and popularity among surgeons, as confirmed by our
physician consultants. Finally, we considered the possibility of separate devices for implant and
putty delivery. Observing the prior art, we saw a gap in dual devices and thus decided our best
market value would come from a design that incorporates both.
We generated a list of needs for our device that included essential components as well as
peripheral perks to evaluate our brainstormed solutions. There were eleven categories (aesthetics,
anatomy, all-in-one, clinical integration, functionality, innovation, material, operability,
prototyping, safety, and size) with 54 subordinate needs. Each need was ranked on a scale of 1 to
10, with 1 as minimal importance and 10 as utmost importance. The rankings represent averages
of each of the team member’s individual rankings. Of note, the top six needs are that the device:
Will not harm the patient, fits the implant, preserves the structural integrity of the implant,
releases the implant, is sterile, and is minimally invasive. Thus, the top needs of the device
cover the main stakeholders, namely the doctor, patient, and by corollary, LifeNet Health.
After coming up with 99 preliminary ideas, we began to eliminate choices to narrow
down to a reasonable number for evaluation. We evaluated the surviving designs in the context
of our needs by giving each design a score from 1 to 10 for how effectively it addresses each of
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the criteria. This number was then multiplied by the importance of that need and those numbers
were added together for each design. These raw totals were calculated for each design by each
team member and then normalized as a percentage of that evaluator’s average raw total awarded.
To generate our final ranking of the designs, we then averaged the normalized scores.
Device Design: Concept Selection
Our top-rated design, the Tri-Grip, addressed both the insertion of the implant, as well as
the application of the putty. As seen in Figure 2, the implant is held on the top by a metal blade
and on the bottom with two additional metal blades. The top blade can pivot vertically to
accommodate varying implant heights. The bottom blades can separate laterally for varying
implant widths. Thus, the surgeon could use the same device for implants of various sizes. In
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addition, the bottom blades would be curved about the axis of the tool so the implant would not
slip out the sides of the device. The top blade would contain a channel within the body and a
syringe tip at the end to house and dispense putty. The syringe tip allows the surgeon to fill the
disc space with putty at any time without removing the insertion device.
The Tri-Grip satisfies most of the criteria by which we judged the competing designs,
such as quick insertion, putty delivery, ease of use, and safety. By incorporating three blades as
opposed to two found in other devices, we increased the protection of both the internal implant
and the surrounding external anatomy. Furthermore, our device increased the surgeon’s control
of the implant and eliminated the concern of the implant slipping out the side of the device.
Device Design: Mechanism Design
Next, we brought the Tri-Grip tip design into a feasible, full concept by modeling the rest
of the device with SolidWorks v.2013. Figure 3 shows the first version of what is now dubbed
the Viper. The design incorporated the Tri-Grip tip with a handle. The bottom blades connected
at the front of the handle and pivoted like a pair of scissors. The handle was parallel to the blades
and contained a gap for two activators. The top button was intended for implant insertion, while
the bottom trigger delivered putty. The top blade contained a port for the surgeon to position a
syringe. This design did not contain specifics on the mechanics of the delivery.
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After speaking with surgeons and consultants, we concluded that the linear device would
not be ideal for our application. In order to retain one-handed operation and dual delivery, we
changed the handle to a perpendicular grip. Our handle was inspired by a motorized pipette filler.
We decided to incorporate the shape of the filler with the blades to arrive at the current shape in
Figure 4. To make the device
functional, we designed a gear
system to translate between
button presses to implant
delivery. Figure 5 shows the
system of spur gears internalized
in the handle.
After rapid prototyping
the device and soliciting feedback
from surgeons, we redesigned the
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Viper to the model in Figure 6. Additionally, the gear system was updated to the current
composition in Figure 7. At first, the gears were spur gears, but were changed to helical in order
to strengthen the assembly. Starting from the bottom, the two buttons propel the implant down
the channel in opposite directions, the top button pushing the implant toward the disc space. Gear
5 between the buttons mediates this action and preserves the ratio of the buttons. Gears 2-5 are
identical in design; each has a smaller gear attached to a larger gear, creating a 1:2 gear ratio.
Gears 4 and 5 are in parallel, while gears 2-4 are in cascade. This arrangement creates a 1:8 gear
ratio. Gear 1 transfers the rotational energy from the gear system to the plunger. Below are
detailed descriptions of the various features of the current iteration of the Viper.
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Viper Features: Ergonomic Handle
The handle grip was initially designed to emulate pipette filler in size and shape. The
handle of the device needed to feel natural and familiar for the surgeon. The two main parts of
the handle, pictured in Figure 8, are the top barrel and the lower grip (where the hand is placed).
The full handle has a metal cage in between the plastic casing in order to provide stability and
structure to the device. The grip of the handle has grooves for the surgeon’s fingers so it can be
held firmly, yet comfortably. While the dimension of the grip was based on the pipette filler,
improvements can be made to better suit individual surgeons’ hand sizes. The back of the grip is
rubberized for comfort during the surgery. Since the surgeon will be putting the most force on
the back of the grip, rubber must be placed there to limit the pressure placed on the surgeon’s
hand. In this current iteration, there are two buttons, one to push the plunger out between the
blades and the other to retract the plunger within the barrel of the handle.
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The handle itself is the housing for the mechanical components, including the gears,
buttons, plunger, and ramrod. Built into the handle on both sides are holes for the gear axles.
They are given a wide base which provides support around the gear axle so that each gear rotates
without wobbling off its axis. The axles are built into the gears for this iteration to minimize the
amount of small, fragile parts from the 3D printer. Built-in axles are more structurally sound and
provided adequate results for our second prototype. The buttons and the plunger each move
linearly along a built-in track on both sides of the handle. The fit between the two sides of the
handle was adequate for this iteration, but can be improved in the future by using more accurate
clearance values and more precise 3D printing techniques. Two windows are cut out of the left
side of the handle: one for the cartridge and one for the ramrod to slide. The front window is in
line with the plunger to provide access for the tip attachments and cartridges. In future testing,
placement of the front window may be altered based on surgeon feedback or preference.
Viper Features: Blades
The blades on the device, depicted in Figure 9, are used to create a pathway for implants
and putty to travel into the intervertebral disc space. The top blade can move up and down to
separate the two vertebrae allowing for the insertion of an implant. This is done passively as the
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implant is being delivered by pushing out the implant, however it can also be done actively using
a screw located on top of the device for added disc separation. This separation creates room for
the implant to be inserted and prevents the degradation of the implant by strong friction from the
vertebrae. The two bottom blades also begin closed, creating a small frontal profile and allowing
the device to be inserted into the disc space via the small window created in a TLIF. The bottom
blades are held together by a spring that will resist torsion. These blades separate passively, like
the top blade, as the plunger is pushed down, creating space for the implant to be delivered. The
three blades allow for better containment and protection of implants as they are delivered. This
ensures the structural integrity of the implant is maintained throughout the delivery process.
Viper Features: Plunger and Ramrod
The plunger is the driving force of the implant. Activated and controlled by the buttons
located at the bottom of the handle, the plunger pushes the implant out of the device, separates
the blades, and delivers the implant to the disc space. As seen in Figure 10, the plunger functions
as a rail for the gear system and contains helical teeth on top. The channel within runs down the
plunger. The wall at its thinnest is 0.5mm. The face of the plunger is 6.35mm in diameter (not
including the helical rack on top of the plunger). The tip of the plunger is cut like a puzzle piece
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to enable attachment of different tips. We decided to create a modular device to account for
different surgical substances, such as implants of varying shapes and sizes, bone putty, or fluids
such as bone marrow aspirate. These will be discussed in the next section.
The ramrod rests in and slides through the inside of the plunger. The component expels
putty by pushing the sliding button on the side of the device. When the plunger is fully extended,
the ramrod can slide to the end of the plunger to push out any putty contained in the channel. It
could also be used to interface with a tip attachment. The ramrod functions independently of the
plunger, so the plunger can still be used for implant delivery without the ramrod.
Viper Features: Side Window, Cartridge, and Tip Attachments
To ensure that this device can be
used for implants of varying sizes as well
as the delivery of fluids, the tip of the
plunger can be changed. The side
window, in Figure 11, allows the user to
switch or remove these tips if necessary
without removing the device from the
surgical site. First, a tip holding the
implant, as shown in Figure 12, would be used for implant insertion, however there would not be
an opening for the putty or liquid to be delivered down the plunger due to size constraints. This
tip contains a static back that connects to the plunger via a puzzle piece end as described
previously. The front of the piece contains another connection point for two sliding walls that
grip the implant. The side grips have wedged tips to enable the surgeon to slide the implant
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between them. The grips are held in tension via a spring within the connection channel. A ledge
on each of the grips keeps the implant from the top of the component. As the surgeon delivers
the implant, the implant grip slides down the shaft and contacts the top blade. The blades form a
rectangular channel, so the implant will be shielded by the blades on at least three sides;
exposure of the bottom face of the implant will depend on the implant height. At the end of the
top blade, a complementary wedge extrudes down and fits between the space created by the
implant grips and the ledges. As the implant grips collide with the wedge, they are forced open
and release the implant. The implant then slides down into the space created by the gap in the
bottom blades. Once the implant is fully inserted, the plunger can be retracted and will not take
the implant with it. In a different function, the surgeon may opt to deliver putty prior to or
following the delivery of the implant. In this case, the surgeon would insert the device and the
plunger could then be pushed back out, lining up the window with the gap in the plunger. This
window to the inside of the plunger is where the surgeon would insert the putty cartridge. This
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cartridge, pictured in Figure 13, is comprised of two parts. The cartridge case resembles the
holder for a VHS tape. The case has an open side as well as a hole at opposite end of each
perpendicular face to the plunger. This hole is just large enough to allow the ramrod to slide
through. With the case inserted, the surgeon can insert putty into the space, either through a
purchased syringe or an autograft-concocted mixture. Then, the other piece of the cartridge, the
cartridge insert, compacts the putty into the plunger. The cartridge insert forces the putty into the
channel of the plunger and, when fully inserted, creates a completed tube with the end of the
cartridge case. The surgeon then activates the ramrod as forces the putty down the plunger
channel. The cartridge can be easily refilled if the surgeon wishes to deliver more putty after
pushing the ramrod down the channel. Figure 14 shows drawing views of the Viper to display
the dimensions and orientation of the device.
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Testing and Validation
Testing of our device will include how surgeons handle and manipulate the device, as
well as the functionality and performance of the device. Our primary prototype is functional and
has provided a basic understanding of how the device will be handled by surgeons. Ergonomics
and handling have been validated with our first and second prototypes with some surgeon
feedback. Functional and performance testing will be performed as new prototypes are
developed with functionality as the main focus. We constructed a spine model, shown in Figure
15, to simulate the basic anatomy and surgical procedure of the TLIF, and we plan to test our
prototype on this model. Moving forward, we will progress to animal and cadaver testing with
surgeons through LifeNet Health.
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The 3D printer plastic was sufficient for presenting and visualizing our CAD drawing.
But the model does not allow for the testing of some physical characteristics of the device. One
testing procedure would be finding the pressure required to push the putty through the plunger,
and the pressure exerted on the walls of the plunger. The mechanical properties of the trigger and
plunger will also be tested to find the force required to move the plunger between the blades and
the force needed to push the trigger. Strength of materials for the device will be tested in a real
surgical environment in order to set the minimum requirements for the device.
Conclusions and Future Work
One of the main concerns with the Viper design is that the two button system which is
operated with one finger at a time may not be sufficient for producing the force necessary to
deliver the implant. One solution which would coincide with the handle grip is having one large
trigger with the grip being the same shape as the handle, as seen in Figure 16. The fingers will
have a half-loop surrounding the fingers, similar to scissor handles. This allows the surgeon to
move the trigger back and
forth effortlessly. Power
generation from four
fingers is greater than one,
and the pressure placed on
each individual finger is
reduced. This trigger
mechanism would also
provide a curved-rack
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interface by which the plunger can be pushed. A reduction in the 1:8 gear ratio would lessen the
pressure placed on the gears and eliminate some space currently occupied by the gears within the
handle body. Two surgeons approved of the concept, and would trust it more for power
generation than the two-button approach. A second option for the trigger would be a ratcheting
mechanism. This would allow the surgeon to use multiple pumps to deliver the implant and ease
the implant into the disc space in an iterative fashion. However, this would entail a redesign of
the plunger retraction, as pulling the grip out would no longer result in plunger movement.
Another way to improve the device is to rotate the grip so the handle is perpendicular to
the superior-inferior separation of the blades. This would allow the surgeon to stand comfortably
to the side of the patient and still be able to deliver the implant. As the device is currently
designed, the surgeon has to lean over the patient to handle the device.
There was also concern about the blades not being long enough for the surgery. They do
not need to be lengthened tremendously, since there is not much space separating the disc space
from the outside of the patient when compared to an anterior approach, but there is still the issue
of some patients being larger than others and thus requiring longer blades.
This device is currently designed to be only used for TLIF spinal surgery. However, one
of the surgeons we interviewed proposed the usage of this device in joint fusion and fracture
repair for its small tip size and putty delivery mechanism. This will be especially helpful for
patients suffer from traumatic, compound bone fractures that are hard to heal with the currently
used metal rods and screws. In future design iterations, this can be considered to broaden the
impact made by this device and increase the likelihood of clinical implementation.
Over the past year, we developed a spinal implant delivery device from scratch given
LifeNet Health’s implant specifications and surgeon preferences. We focused on the TLIF
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approach to take advantage of the popularity of the procedure among surgeons. To differentiate
our device from the competition, we incorporated a putty delivery system. The putty and implant
delivery mechanisms would be independent of each other, but able to function in sequence with
ease. Additionally, this design affords the surgeon flexibility in what order they perform aspects
of the surgery. Finally, the Viper allows for future modifications, anticipated or unforeseen, to be
integrated into the device with minimal redaction of previous work by exploiting the modular
components.
Acknowledgements
The authors would like to express their sincere appreciation to Dr. Joseph Gjolaj, Dr.
Francis Shen, Dr. Adam Shimer, and Dr. Joseph Li for their valuable insight to spinal fusion
surgeries and the current tools that are available as well as Dennis Phelps for his insight into the
mechanical aspects of the device.
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Appendix B: Design Honorable Mentions
The Oyster – This design incorporates a putty
delivery system with the insertion device. The
implant will be placed inside the body space
which will then be filled with putty. When the
device reaches the vertebrae, the top and bottom
piece will open up to retract the vertebrae apart
and the implant together with the putty will be
pushed into the disc space. The top and bottom
piece will then close up for smooth exit.
The Trigger Mechanism - This idea illustrates a
mechanism that was initially considered during the
original brainstorm process. Pulling on the trigger
would drive the plunger down the device and deliver
the implant into the disc space. This design is being
reconsidered now that surgeons have expressed
concern about using a single finger trigger to deliver
enough force to deliver the implant.
The Wrench – This design incorporates a putty
delivery system with the insertion device. The
innovation of this device is the implant-grabbing
branches are filled with putty. When the device
inserts the implant into the disc space, it starts
squeezing out putty from two branches. At the
meantime, the branches retract back to release the
implant. Thus, when the process is finished, the
implant will be circled with putty around it in the
disc space.
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The Sandwich – This design incorporates a putty delivery system
with the insertion device. The Sandwich, has a movable middle
piece the can slide back and forth. After wrapping the implant with
putty using a separate molding device, the entity will be loaded to
the middle piece, which then slides back to protect the implant in
the device bodyspace. When the device reaches the disc space, the
top and bottom space will retract the two vertebrae apart and middle
piece slides into disc space. The movable rod at the center of the
middle piece will pop out the implant and putty to the disc space.
